submodule--helper: introduce add-config subcommand

submodule--helper: introduce add-config subcommand

Add a new "add-config" subcommand to `git submodule--helper` with the
goal of converting part of the shell code in git-submodule.sh related to
`git submodule add` into C code. This new subcommand sets the
configuration variables of a newly added submodule, by registering the
url in local git config, as well as the submodule name and path in the
.gitmodules file. It also sets 'submodule.<name>.active' to "true" if
the submodule path has not already been covered by any pathspec
specified in 'submodule.active'.

This is meant to be a faithful conversion from shell to C, although we
add comments to areas that could be improved in future patches, after
the conversion has settled.

Signed-off-by: Atharva Raykar <raykar.ath@gmail.com>
Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Shourya Shukla <periperidip@gmail.com>
Based-on-patch-by: Shourya Shukla <periperidip@gmail.com>
Based-on-patch-by: Prathamesh Chavan <pc44800@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>